This invention relates generally to thick-film resistor networks and, more particularly, to thick-film resistor networks used for lightning surge protection in telecommunications systems.
Lightning surge resistor networks are well-known in the telecommunications industry. Such networks, which typically comprise a thick-film resistor formed on a ceramic substrate, are used to terminate individual telecommunications lines. In addition to providing impedance matching, such networks protect the telecommunications circuitry from over voltage conditions that can result when a lightning bolt strikes on or near a telecommunications line.
Ordinarily, lighting surge resistor networks are designed to withstand a typical over voltage condition without incident. Sometimes, however, a non-typical or severe over voltage condition occurs that is sufficient physically to destroy the lightning surge resistor network. This can happen, for example, when a particularly strong lightning bolt hits the telecommunications line. Or it can happen when an alternating-current power line strung adjacent the telecommunications line falls onto the telecommunications line and delivers the full power line voltage to the telecommunications line. When such severe over voltage conditions occur, it is not unusual for the thick-film resistor element in the lightning surge resistor network to open in a shower of sparks, much in the manner of a fuse. Although such self-destruction by the resistor network is effective to protect the downstream telecommunications equipment, the resulting "fireworks" can be problematic. Molten matter sprayed onto adjacent electronic components can cause short circuits, and uncontained arcing creates the potential for fires. Good design practice requires that such adverse consequences be avoided.
One known approach to avoiding such adverse consequences involved coating the thick-film resistor with an epoxy coating. The coating helped avoid spraying adjacent components with ejected matter when the thick-film resistor opened. Another known approach was to scribe lines into the ceramic substrate on which the thick-film resistor was deposited. The scribed lines formed stress concentrations that would crack the substrate and thereby open the resistor when the resistor overheated in an over voltage condition. Although effective in allowing the resistor to open in a controlled, non-dramatic manner, the approach did not work fast enough in extreme (i.e., 600+ VAC) over voltage conditions. Under such conditions, uncontrolled arcing could still occur.